1. Field of the Invention
This invention relates to image sensors, in particular having arrays of image sensing pixels, for example for use as solid state x-ray imaging devices.
2. Description of Related Art
There is significant interest in developing solid state x-ray imaging devices, to replace the image intensifiers currently used in hospitals.
Various pixel configurations have been proposed in which each pixel comprises a light sensitive element, such as a photodiode, and at least one switching device. For example, one known pixel design comprises a single thin film transistor (TFT) and a photodiode. During an exposure period, the TFT is turned off so that the photodiode is isolated. Incident light causes a minority carrier current to be produced, which causes the self parasitic capacitance of the diode to be discharged. During the next readout, the capacitance of the diode is reset and the change in charge is detected by the amplifier.
It has also been proposed to include an additional storage capacitor within the pixel configuration, in order to provide gain. The storage capacitor charge can then be detected by the readout amplifier.
One problem with this approach is the area required for the storage capacitor which can limit the possible resolution which can be achieved. There is, however, a need to provide pixel gain, as this improves the signal to noise ratio of the image sensor pixel.
According to the invention, there is provided an image sensor comprising a plurality of pixels, each pixel comprising:
a light sensor element, a sensor voltage across the element varying depending on the light incident on the element; and
first and second transistors, the first and second transistors being connected in series between voltage supply lines, a gate voltage on the first transistor being dependent upon the sensor voltage so that the current flowing through the first transistor is a function of the sensor voltage, and wherein the gate voltage of the second transistor is supplied by a feedback circuit which provides that the current through the first and second transistors is substantially equal, and wherein the output of the pixel is the gate voltage of the second transistor.
The image sensor design of the invention avoids the need for storage capacitors, because the voltage across the light sensor element is measured, rather than the flow of charge through the light sensor element during a recharging process.
Preferably, the first and second transistors have different current versus gate voltage characteristics, so that the pixel provides gain. Again this is achieved without the need for storage capacitors which can occupy a large area of the image sensor substrate.
Preferably, the feedback circuit includes an open loop amplifier, so that a minimum current is drawn, which enables the current through the first and second transistors to be kept substantially equal. The input to the amplifier may be the junction between the transistors, and the output from the amplifier is then coupled to the gate of the second transistor.
The image sensor preferably comprises rows and columns of pixels, with one amplifier provided for each column.
The light sensor element preferably comprises a photodiode, which may be coupled between one of the voltage supply lines and the gate of the first transistor. To provide an initial reset function, a switch may be provided which enables a predetermined voltage to be applied to the gate of the first transistor, thereby charging the photodiode to a predetermined level.
The invention also provides a method of measuring light intensity of an image to be detected using a plurality of light sensor elements each forming a pixel of an image sensor, a sensor voltage across the elements varying depending on the light incident on the elements, the method comprising:
applying the sensor voltage to a gate of an associated first transistor thereby causing the current flowing through the first transistor to be a function of the light sensor voltage;
supplying a gate voltage to an associated second transistor using a feedback circuit which provides that the current through the first and second transistors is substantially equal; and
measuring the gate voltage of the second transistor as output.